


Heated Pursuit

by SoloShikigami



Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap (Undertale), Incest, M/M, Mating Cycles/In Heat, Overstimulation, Underswap Papyrus (Undertale), Underswap Sans (Undertale)
Language: English
Status: Completed
Published: 2020-05-05
Updated: 2020-05-05
Packaged: 2021-03-02 18:07:51
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,247
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/24021076
Author URL: https://archiveofourown.org/users/SoloShikigami/pseuds/SoloShikigami
Summary: The skeleton brothers always helped each other when they went into heat, though Papyrus' cycle seems to be much stronger this time. Will Sans be able to keep up this time?
Relationships: Papyrus/Sans (Undertale)
Comments: 5
Kudos: 86





	Heated Pursuit

**Author's Note:**

> Written for Sanspar, originally posted in September of 2018!

“I AM GOING TO MY ROYAL GUARD TRAINING!”

“Mm’kay bro.”

“WATCH OVER THE HOUSE WHILE I AM GONE!”

“Y’know I’ll keep an _eye-socket_ on it.”

“HMPH. WELL, KEEP THAT MANGY PUP AWAY FROM MY FOOD LABORATORY!”

“Dunno, I’m kinda _doggin_ ’ it today, myself.”

“MWEH! NO PUNS! _AND PICK UP THAT SOCK_!”

“I will, I will, now _shoe_.”

“UGH!”

With an indignant huff, Sans slammed the door. Papyrus chuckled from his place lounging on the couch.

“Sans is so cool,” Papyrus murmured to himself. “To put up with such a lazy bone as myself. I should do something, but… eh, maybe after a nap.”

The tall skeleton settled himself into the couch, making himself comfortable, and shutting his eyes and expecting to slip easily into slumber.

But sleep wouldn’t come.

He lay still for a while, about half an hour, but still he remained awake.

No, indeed, something was bothering him. After some time, his eyes flew open and the need to nap was gone quicker than a snowman in Hotland, and he all but leaped off the couch. He walked over to the kitchen, but then he felt confused as to why he was there. He glanced around, itching for something to do, but the kitchen was spic and span as Sans was a very thorough cleaner.

But he didn’t want to clean anyhow.

Papyrus moved back into the living room, glancing at the sock and considering picking it up for a moment, but something told him that picking it up wasn’t going to satisfy this sudden restlessness that was taking him over.

Instead he settled for pacing, rubbing his arms as he felt chilled and overheated at the same time.

What was going on? Was he getting sick? Why was his magic acting up? Was this some weird side-effect of all the resets? From working with Gaster and the machines? What to do, what to do, what to do…..

Wait, the room smelled like honeysuckles?

Papyrus stopped in his tracks.

“Oooohhhh shit,” Papyrus said. “I better warn Sans.”

Papyrus ran out of the house, leaving his phone stuck between the cushions of the couch, the thought of calling Sans never once crossing his mind.

~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~

Sans hummed a little as he made his way through Waterfall, running over the last lesson in his mind so it would stay fresh for today. Not only was he hoping to showcase how well he had practiced dodging, he also wanted to impress the captain with a new taco-making technique that he had spent the last week developing.

He was almost so lost in thought that he nearly missed the rustling in a nearby bed of Echo Flowers.

“Mweh? Hello? Is someone there?” Sans called.

“Is someone there?” an Echo Flower repeated.

But other than that, silence, so Sans continued walking, until there was another rustle in the next bed of Echo Flowers. Sans stopped, sighing, and putting his hands on his hips as he turned to the sound.

“ASRIEL! IF THAT IS YOU, I _WILL_ TELL YOUR PARENTS THAT YOU ARE HERE!” Sans said with authority.

The rustling didn’t really stop, but seemed to slow.

“Asriel, I know you are a huge fan of Alphys, she is quite cool and popular after all,” Sans explained as nicely as he could as he started going towards the rustling. “But we are going to be training and it will be very dangerous for you to be so-!”

Sans let out a gasp when an orange light flashed between the blue petals; that was most definitely _not_ Asriel. Before he could make any sense of what it was, it moved forward fast and Sans was off and running, any rational thought fleeing as fast as he was.

“Hey! What gives?!” Sans cried out when he felt that something was hitting the ground as he ran, just barely missing his heels with each step. Something whizzed to his right, so he ducked and rolled to the left.

Sans only had a second to take a breath before he could feel whatever chasing him wasn’t done yet, and he knew he had to move. He scrambled to his feet, glancing around, and his eyes caught the door to the next room and he bolted for it.

He skidded to a stop after a few feet as the darkness around him began to swallow him up.

“Oh, drat,” Sans groaned. “The Mushroom Room.”

There was a group of mushrooms glowing pale blue deep in the darkness giving off a pulsing light that formed a path, but he didn’t know where the next part of the path was.

As magnificent as he was, he never could remember paths well.

But the chase was still on, as he felt more than saw another attack land just behind him. With a cry of surprise, he rushed forward, picking the longest part of the path.

“ _Maybe this is Alphys trying to test me, that’s gotta be it,_ ” Sans thought to himself as he ran. “ _Why else would anyone want to attack me like this?_ ”

The path was ending, and Sans leapt, hoping to touch the next group of mushrooms to light the next part of the path, and breathed a sigh of relief when his fingertips brushed along the soft head of a mushroom. He rolled onto his back in preparation to get up when something hit the ground close to his shoulder, stunning him still. He glanced over and saw that it was a bone.

“What the-?”

Out of the darkness loomed a tall figure, and while Sans was quick enough to piece together that it was Papyrus, he still gasped when he brother shuffled into view.

“YOU SCARED THE MARROW OUT OF ME!” Sans shouted.

“Nyeh, heh, heh, sorry bro,” Papyrus said, though not sounding or looking sorry in the least.

“YOU SHOULD BE! WHAT IF I MISTOOK YOU FOR A HUMAN AND ATTACKED YOU?”

“Aww, you wouldn’t hurt your dear big bro, would you?”

“Well, no, but- what are you doing?”

Papyrus straddled his legs and leaned over him.

“I just wanted to play a little game, Sans.”

“You picked an odd time for a game,” Sans said slowly. “What game did you have in mind?”

“Mmm, tag, maybe?”

“Hmm, I am a little ahead of schedule, and it may be a good warmup, let me just text Alph-! Hey!”

Sans lightly slapped Papyrus on the top of the skull, as he had moved down Sans’ body while he was taking out his phone and nudged his shirt up a little, and his tongue had just lapped at a lower rib. Papyrus let out a low growl in response, but since Sans was busy texting, he didn’t catch it.

“And sent! Okay! You know, I’m proud of you for getting some exercise,” Sans said with a grin. “Who is ‘It’ first?”

“Me,” Papyrus said, moving back down to lift Sans’ shirt again.

“Okay!”

Sans moved to stand but Papyrus moved to hold him down.

“No!” Sans scolded, bopping Papyrus on the head again. “You have to give me a ten second head start! It’s only fair!”

Papyrus snarled at the second smack, but Sans just gave him a stern look.

“Ten,” Papyrus muttered.

“That’s better, but I should at least be up before you start,” Sans complained as he finally moved out from under his brother.

“Nine.”

“Come on, Papy, you’re cheating!”

“Eight.”

“Then again, I guess I don’t need that much of a head start.”

“Seven.”

“Since I am The Magnificent Sans, after all.”

“Six,” Papyrus got to his feet.

“Besides, you’re lazy! Mweh heh heh!”

“Five,” Papyrus stepped forward, looming over Sans, and staring intently down at him.

“Uhm, I mean that in the best way possible.”

“Four.”

“I, uh, I guess I better go,” Sans said, starting to feel unsure.

“Three.”

“Okay then!”

“Two.”

“Catch me if you can! Mweh heh heh!”

“One.”

Papyrus waited a moment as Sans took off.

“Nyeh, heh, ready or not, here I come.”

Sans would never swear out loud, but he was in his head as he tried to pick out the path and put as much distance as he could between himself and his brother. However, even as he found the next group of mushrooms to activate the next part of the path, he could hear Papyrus’ footsteps behind him.

“ _How is he so fast today?_ ” Sans thought.

The thought was quickly driven out again when Papyrus began flinging bone attacks at his feet again.

“HEY!” Sans protested, but then squeaked when an attack flew over his head.

Thankfully, they had gotten to the end of the Mushroom Room, but Sans groaned in frustration when he remembered that the Lantern Room was next. He was able to make it to two lanterns, dodging the attacks his brother threw with some ease, but he tripped when he went for the third lantern.

Sans half expected to fall flat on his face, but something gripped under his arms, still moving forward, and instead of his face hitting the ground, he felt himself being pressed against a wall.

“I win,” Papyrus murmured near his jaw with a small chuckle.

“Yeah, sure,” Sans panted. “But, what gives, Papy? You’re acting really weird!”

“I figured I would help you train a little.”

“While that is very generous of you, it’s unusual.”

“Mmm, maybe.”

Papyrus moved closer, pressing Sans more into the wall. Sans realized that he was only supported by Papyrus’ weight against him, pushing him harder against the rough rock. Papyrus was gently nuzzling the side of his skull, and Sans could swear that he was purring.

“P-P-Papyrus?” Sans stammered uncertainly. “A-are you okay?”

“I’m fantastic, Sans,” Papyrus murmured against his skull. “You were my prey, today, and it seems I have captured you.”

Sans gasped as he felt a hand slip up his shirt, fondling his ribs, another hand curving over his hipbone, brushing over the front of his shorts. He kicked slightly, but being pressed against the wall as he was, he didn’t have much leverage or ability to move. He then slowly realized that Papyrus felt very warm, he could even feel him through their clothes. There was also the flowery scent of honeysuckle coming off of him in waves, and Sans slowly started putting two and two together as a low rumble came from Papyrus’ chest.

“I don’t know if I can control myself, Sans, it’s bad,” he whispered, as if helping to put the final piece of the puzzle together. “I need you, please.”

He pressed his body against Sans, letting out a low moan. Sans’ face lit up in a bright blue blush as he could feel something hard, but slightly giving, pressing into his tailbone.

“O-okay Papy,” he said. “I, I understand, b-but, please, not here?”

“Why not?” Papyrus purred, rubbing himself against Sans again. “No one is here, and it’ll be dark soon, so no one will see us.”

“B-but they can h-hear us!” Blue struggled under the bigger skeleton.

Papyrus’ hand slipped under the waistband of his shorts, and Sans’ voice was cut off with a small gasp. He began scratching at the wall as the long, familiar fingers pressed on sensitive, familiar spots.

“Then everyone in the Underground will know that you belong to me.”

Sans tried to protest more, but Papyrus’ fingers brushed along his tailbone and spine, and his magic formed quicker than he would have ever liked to admit. Papyrus’ heat was beginning to affect him, he realized, as his felt lost in a haze of heat and honeysuckle.

“C’mon, Sansy, let me hear you,” Papyrus murmured, biting his neck gently as his fingers wrapped around the forming cock.

“Nnng, mweh, Papy, no,” Sans protested.

“We both know you could stop me if you really wanted to,” Papyrus said clearly.

Sans grit his teeth; this wasn’t the first time either of them had been lost in the frenzy of heat, and in Papyrus’ case, it was established that hearing the word ‘reset’ would bring him back to reality for long enough to make sure they both proceeded safely. Sans didn’t know why that word in particular, but he did know that it worked.

And while he was embarrassed, he was also very turned on and wanted to see how far this would go.

“May I take you silence as consent?” Papyrus asked.

Sans nodded, and Papyrus resumed slowly stroking him as he pressed in from behind.

“You’re so beautiful and strong, Sans,” Papyrus murmured. “And you’re mine, all mine.”

Sans whined, stuck between pressing back on his brother and the wall. Papyrus growled, biting somewhat hard on Sans’ neck to keep him still. Sans whimpered and stilled, and he was rewarded with loving licks on the bitemark.

It didn’t take much longer for Sans to realize he was reaching his peak and his hands scrambled against the rock again.

“Pa-Papy, I-I’m close, st-stop, please,” Sans panted.

“Mmm, nah, if you’re close, might as well finish,” Papyrus murmured. “I want to leave my mate satisfied, after all.”

“I am, I am, Papy, please!”

“Cum for me, Sansy, cum for your bro.”

Sans couldn’t hold back, and he pressed his gloved hand over his mouth to muffle his cry, his body shaking as his orgasm took over, feeling something warm and wet trickle down one femur.

“Such a good boy,” Papyrus praised, still slowly pumping Sans, milking him of what he had to give, then moving his hand to his backside, fingers seeking his entrance. “But we’re not done yet.”

“Mm, no, no, please, Papy, you can do what you want, just, just please, let’s do it at home!” Sans pleaded, his soul beating hard at the prospect of being found now.

“May I really?” Papyrus asked, his voice seeming to sober. “Can I do what I want?”

“Yes, I promise, just please take us home.”

Papyrus chuckled darkly. “As you wish, Sans.”

Sans wondered if Papyrus knew what it did to him to hear his name being spoken like that, and before he knew it, his back was being pinned down roughly on the thinly carpeted floor of Papyrus’ bedroom with Papyrus all but humping him through their clothes.

“My room,” Sans began to protest, but it was cut off by another sharp bite at his neck.

“My rules,” Papyrus growled. “Everything tonight is mine.”

Sans was too shocked to react as his armor was peeled off and his shirt yanked over his head. Papyrus bent and captured his mouth in a deep kiss as his hands scratched lightly over his ribs and spine, brushing along those spots again that made Sans gasp and squirm. He was all but helpless, his mouth being ravaged, and his body still trembling slightly from his orgasm. Papyrus moved down his body quickly, nipping at a few ribs until he stopped at the little belly Sans had formed. Sans blushed even harder; he was always a touch embarrassed about the bit of chub his magic would produce in his middle, but Papyrus always showed how much he adored him no matter what.

“So beautiful and sexy,” Papyrus murmured against the blue flesh, licking little patterns as his hands kneaded lightly. “Fuck, I am so lucky to have you.”

“L-language,” Sans tried to scold, though it was difficult to take a stuttering scolding seriously.

“What about it?” Papyrus murmured, dropping kisses and small bites along the magic as his hands began working on undoing Sans’ pants. “You don’t like hearing how sexy I think you are? I thought you’d like to know all the things I want to do to you, how many ways I want to love and pleasure your body.”

Sans started to whimper, his own hands clawing at the carpet beneath him, already feeling the waves of pleasure overtaking him.

“Papyrus, d-don’t,” Sans murmured, trying to get some semblance of control as his hands moved to cover himself.

“Aw, c’mon bro, don’t be like that,” Papyrus chuckled, taking Sans’ hands gently but firmly away, moving back up to nuzzle his belly again. “I love every inch of you, every angle, every curve, everything that is you.”

Papyrus moved back up his body, leaning down to look Sans in the eyes. Sans looked up, a little surprised to see such clarity in his brother’s face despite his heat. Papyrus kissed him gently.

“I love you, Sans,” Papyrus said, pressing his body down on him again.

Sans couldn’t help groaning; Papyrus’ body felt like it was on fire, his own desire had been stirred since Waterfall and his body was already wanting more, and he wanted everything Papyrus was willing to give.

“I love you too, Papy,” Sans panted. “P-please, do what you n-need to do.”

“Really?”

Sans finally moved his hands to gently grip Papyrus’ face and leaned up to kiss him deeply.

“Yes,” he whispered when they parted. “Really. I want this.”

With a possessive growl, Papyrus bit Sans’ neck again, drawing another cry from the small skeleton.

“Mine,” Papyrus growled before moving back down between Sans’ legs and yanking off his pants.

“P-Papy, w-what are y-you going to do?” Sans stammered, a little shocked by the sudden return to such rough intimacy.

Papyrus only let out a deep, dark-sounding chuckle that made Sans shudder. He reached up and firmly raked his fingertips from the bottom edge of his ribs, down his belly, and across his hips and down his legs. Sans’ back arched and he clawed at the floor again. Papyrus curved his arms under and around Sans’ femurs, holding him still as he nuzzled his thick hard-on.

Sans looked down, panting, squirming, the blush on his face growing as Papyrus slowly leaned down to lick the length of his cock slowly, cleaning off any leftovers from earlier, swirling his tongue around the head, swiping across the weeping slit.

“Nnng, Papy,” Sans groaned.

“Mm, yes, Sans, let me hear more,” Papyrus murmured before swallowing Sans’ rod down again.

Sans moaned and whimpered, his head tossing, his hands fidgeting, not knowing what to do with them, and finally settled on folding them over his face.

Growling around Sans, Papyrus doubled his efforts, bringing Sans to the edge of his peak and then releasing him. Sans slowly moved his arms to peak through them.

“W-wha- why, why’d you st-stop?” Sans asked.

“Uncover your face,” Papyrus ordered.

Sans’ hands shook as he obeyed. Papyrus released one leg, his hand shooting up to grip Sans’ wrists, squeezing them tightly together and pressing them into his stomach as he returned to pleasuring him. Sans’ small cries and whimpers filled the room, and he could barely warn Papyrus when he came again.

Papyrus purred, enjoying the taste of his lover, pulling back slowly to savor the sight of his spent partner, knowing that they weren’t done by a long shot. He took this moment to push his shorts down, gripping his own cock and pumping it a few times with one hand, his other turning Sans a little onto his side, bringing his leg up to rest on his shoulder, opening him up. He licked the fingers of the other and pressed one to Sans’ entrance.

Sans started at the intrusion, but in his position, he couldn’t move. Papyrus was quick to make him unwilling to move as he pressed his finger deeper into his magic.

“I love how tight you always are,” Papyrus said. “It always feels so good to be inside of you.”

Papyrus pressed a second finger in and Sans’ small whimpers started up again, this time, he was pressing into Papyrus as much as he could from his position.

“Oh, you really like this,” Papyrus chuckled. “That’s my good boy, taking it all so well.”

“S-shut up,” Sans stammered.

“What was that? You never say such things, I must have hit a nerve.”

Another finger was added, and the movements became faster and rougher, hitting a spot inside Sans that he all but forgot about, and between that and already being on edge from two orgasms, a third hit without warning.

“My, my, you must be eager,” Papyrus said. “Very well, I can’t wait any longer anyway.”

Wetting his hand with Sans’ recent released, Papyrus stoked himself a few more times before lining himself up. He thrust into Sans with one slide, making Sans scream.

“T-too much!” Sans cried, tears stinging his eye-sockets.

“I got you,” Papyrus murmured against the legs propped up on his shoulder, turning his head to lick at the bone, making slow, shallow thrusts. “Fuck, you feel good.”

Sans’ head was spinning, and for the time being he felt like there was nothing he could do but simply take what Papyrus was giving. He couldn’t deny that it felt good, though – Papyrus was never this rough before, usually keeping Sans’ general inexperience and more gentle nature in mind. Sans began to wonder if this was how Papyrus really wanted to have sex when a growl and a hand stroking his cock yet again brought him back to the present.

“I want you to think of me and only me,” Papyrus snarled, moving faster.

“Ahhh, Papy,” Sans whined, caught between moving into Papyrus’ motions and lying still.

“I know, you’re gonna cum again,” Papyrus said. “Me too, bro, let’ do this one together.”

Papyrus didn’t change his pace, though he did lift Sans up a bit more, changing the angle just so to hit that deep sweet spot again. Sans let out a somewhat strangled cry and Papyrus grunted as they released nearly at the same time. They were still for a moment, Sans breathing an inner sigh of relief as Papyrus carefully lowered him back to the floor, but vaguely wondered why Papyrus was rolling him onto his front. He felt his arms being pulled behind him, wrists held together again, and before he could form words, Papyrus was inside of him again.

“Mmmm, this feel nice,” Papyrus said. “Warm, slick, you’re still pretty tight, though.”

Sans’ eyes were wide and he couldn’t cry out anymore, so instead just panted and whimpered. Papyrus moved a little slower, but his thrusts were longer and went deeper than before. Sans was also reminded how much bigger Papyrus was compared to him, as he seemed to envelope him, the one hand still holding his hands behind his back, the other gripping the back of his neck firmly, pressing down to keep him controlled and still.

Sans didn’t know how long it took this time, what he did know was that his body was shaking, another orgasm approaching, and he was starting to wonder how much more he could take.

Papyrus groaned, his second release hitting, and he could feel Sans twitch and tighten around him as he came again as well.

“Mmm, good, Sansy, so good,” he purred.

He released Sans’ wrists, but Sans had no energy to move and his arms just dropped uselessly to the floor. He was still quivering, breathing heavily, and only whined a little when Papyrus rolled him onto his back.

Sans looked up at Papyrus, his eyes hazy and unfocused. Papyrus smiled, leaning down to capture Sans’ mouth in a kiss, distracting him as he moved in and entered him yet again. Sans’ small cry was easily swallowed, and Papyrus set a steady pace.

“Love you, so much,” Papyrus murmured. “Such a good boy for me, so good, so proud of you, fuck, you’re amazing.”

Sans wondered if a monster could dust from blushing. Papyrus continued to murmur praises, reaching down to stroke along Sans’ ribs, down his belly, touching him wherever he could. Sans shook with every brush of fingertips, his body overly sensitive and overstimulated.

“P-papy, please,” Sans could barely stammer.

Papyrus growled, hearing the plea in the tone, and in response he wrapped his arms around Sans, pulling him in tightly, protectively, his pace quickening.

It was too much and too overwhelming for Sans, and while he was certain he came again, his senses cut out completely afterwards.

~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~ * ~

“You are so good, so amazing, I love you so much.”

Sans’ eye blinked open at the praises being gently murmured into his skull. The familiar, soft sheets of his own bed were cushioning one side, the almost too-warm pressure of his brother’s body was pressed against his back.

“Sans? Are you with me?” Papyrus asked.

Sans nodded, not trusting himself to speak yet. He was lifted slightly, a cup of water pressed to his mouth and he gratefully drank.

“Should be taking care of you,” Sans murmured, his voice a little hoarse but soothed by the cool water.

“You are, you did let me fuck you senseless, after all.”

“Language.”

“Sorry.”

He still didn’t sound sorry.

Papyrus settled behind him again, adjusting pillows and sheets to make Sans comfortable.

“You’re not done,” Sans murmured, realizing that Papyrus was still very warm, and his cock, while not hard, was still formed and pressing into his tailbone.

“No, but I have control right now, and you need to rest.”

“Mmm.”

“Sans?”

“Hm?”

“You know I really do love you.”

“Mmm hmm.”

“Thank you.”

A kiss was pressed gently on his neck against a bitemark that was left from earlier, sending a small twinge of pain but quickly followed by a pleasant tingle down Sans’ spine. He smiled, giggling a little as he turned over to capture Papyrus’ mouth in a deep kiss.

“The Magnificent Sans is always glad to help. I love you, too, Papyrus.”

**Author's Note:**

> Like what you read? Please check out my social media for more fun!  
> [ My Tumblr](https://soloshikigami.tumblr.com/)  
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